Cameras and other imaging devices often use one or more image sensors, such as a charge-coupled device (CCD) image sensor or a complementary metal-oxide-semiconductor (CMOS) image sensor to capture an image. In certain situations, the charge or signals produced by multiple pixels are binned or combined into a single signal. For example, pixel binning can be used when the output image resolution of a camera is less than the resolution of the image sensor in the camera, or to increase the sensitivity when an image is captured at low light levels. Pixel binning can be performed in the pixel array or after the signals are read out of the pixel array. Summing the pixels after the pixels have been read out of the pixel array does not increase the readout time since all of the pixels still have to be read out. Additionally, noise produced by the readout circuitry is summed with the pixel signals. The additional noise decreases the signal to noise ratio, which can degrade image quality.
Summing pixels in the pixel array can reduce the readout time since fewer pixels are read out of the pixel array. Generally, summing pixels in the pixel array is performed in orthogonal directions. FIG. 1 illustrates orthogonal summing with pixels in an image sensor with a Bayer color filter arrangement. The pixels are identified generically with the letters A, B, C, and D. In orthogonal binning, neighboring pixels in each color plane are binned together forming orthogonal clusters of 2×1 or 2×2 pixels. FIG. 1 depicts four clusters of 2×2 pixels. Four A pixels are summed together in cluster 100, four B pixels in cluster 102, four C pixels in cluster 104, and four D pixels are summed together in cluster 106. However, when the clusters are summed orthogonally, the clusters are out of alignment with respect to each other. This misalignment can produce spatial color artifacts in an image, which in turn reduces image quality.